linux-kernel - Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

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Message-ID: <20190503123126.3a2801be@gandalf.local.home>
Date:   Fri, 3 May 2019 12:31:26 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Joerg Roedel <jroedel@...e.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, stable <stable@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call
 functions

On Fri, 3 May 2019 09:20:55 -0700
Andy Lutomirski <luto@...capital.net> wrote:

> So here’s a somewhat nutty suggestion: how about we tweak the 32-bit entry code to emulate the sane 64-bit frame, not just for int3 but always?  Basically, the entry asm for entries from kernel mode would do, roughly:
> 
> push $0 ;dummy for call emulation
> push %ss
> push $0 ;a dummy for ESP
> push 3*4(%esp) ;EFLAGS
> push 3*4(%esp) ;CS
> push 3*4(%esp) ;EIP
> push %rax
> lea 7*4(%esp), %rax
> mov %rax, 4*4(%esp) ;ESP
> 
> And the exit asm would do a little dance to write EFLAGS, CS, and EIP to the right spot, then load ESP-3*4 into %esp and do IRET.
> 
> Now the annoying kernel_stack_pointer() hack can just go away, since regs->sp is always correct!
> 
> I probably screwed up some arithmetic there, but it’s the idea that counts :)

Yeah, as it will end up with:

 $0 ; dummy for call emulation
 %ss
 $0 ; dummy for ESP
 EIP
 $0
 %ss
 $0

As 3 only gets you over what you already pushed. I think 5*4 is what
you want.

I guess the real question is, what's the performance impact of doing
that? Although, this is only needed for kernel -> kernel exceptions,
which are hopefully a rarity.

-- Steve